The present invention relates to an arrangement for controlling at least one engine valve of a combustion engine via a valve controlled hydraulic circuit.
Engine valve here means an inlet valve or exhaust valve for a combustion chamber of a combustion engine.
The movements of such engine valves are traditionally controlled by a camshaft by means of cams whose profile controls those movements, the camshaft being driven via a transmission by the engine""s crankshaft. With such traditional control, many parameters which are related to the engine valves are not variable relative to one another. For example, the opening and closing times of the engine valves cannot be varied with the engine speed. An engine can therefore not be caused to achieve optimum operation throughout its speed range.
A number of mechanical systems have been developed for achieving a more flexible control of engine valves. Such systems are operationally reliable and relatively uncomplicated but do not result in fully variable control of engine valve lifting heights and opening and closing times.
Electrical systems for control of engine valves have also been developed. In such cases, engine valve movements are controlled by electromagnets. Such systems can control engine valve opening and closing times variably, but partial lifting of engine valves is difficult to achieve. It is also difficult for electromagnets to exert sufficient force to effect valve lifting when the gas pressure in the combustion chamber is high.
Finally, hydraulic control systems for engine valves are known. Hydraulic systems make it possible to vary engine valve opening and closing times and lifting heights. The known hydraulic systems operate at a substantially constant high pressure. A hydraulic pump and a pressure limiting valve are used to impart this high pressure to a hydraulic fluid which is stored in an accumulator. When an engine valve is intended to be moved to an open or closed position, a control valve is switched so that the pressurised hydraulic fluid is led into a circuit to a hydraulic cylinder in which the hydraulic fluid moves the piston. The problem with such systems is that they require a substantially constant high pressure which itself means that such a system involves a relatively high energy consumption. The high pressure and the need for the system to be completely tight entail severe requirements for the seals which form part of the system.
The object of the present invention is to provide an engine valve control arrangement which makes it possible for the engine valve opening and closing configuration to be substantially completely variable and which at the same time entails low energy consumption and does not require expensive seals.
This object is achieved with the arrangement mentioned in the introduction, which is characterised by the pump being designed to circulate a flow of medium continuously in at least part of the circuit during an operating state of the combustion engine, and by the control valve being designed to control according to need the flow of medium circulated by the pump to the power device so that the latter moves the engine valve in a desired direction. The advantage of having a continuously circulating medium flow is that the medium already has a certain velocity in the circuit. Rapid movement of the engine valve in a desired direction is achieved by directing such a medium already circulating to the power device. The pressure drop in the circuit can be kept at a low level by providing the hydraulic circuit with large flow cross-sections. The pump therefore requires relatively little energy for it to circulate the medium continuously in the circuit. As a high pressure is only required for the short periods of time when the engine valves are accelerated by the medium, the arrangement may be constructed entirely or substantially entirely without seals. In addition, components incorporated in the circuit may be manufactured with relatively large tolerances. The possible advantages include the arrangement being relatively insensitive to thermal variations and insensitive to any contaminants in the oil.
According to a preferred embodiment of the present invention, the control valve has at least two possible positions, whereby a first position results in the medium flow being led so that the power device moves the engine valve in a first direction, and a second position results in the power device moving the engine valve in a second direction. For such a control valve to function properly, it needs to be movable quickly between said two positions. The components incorporated in the circuit therefore need relatively large flow cross-sections so that a large quantity of medium can reach the power device without considerable constriction losses. With advantage, the power device incorporates a double-acting hydraulic cylinder with an internal space divided into first and second chambers by a piston which is movable within the chamber and which is connected to the engine valve, whereby the control valve in the first position is intended to lead the circulating medium to the first chamber, and in the second position to lead the circulating medium to the second chamber. The circulating medium may thus alternately be led by means of the control valve to the first and second chambers of the hydraulic cylinder in order to move in a desired direction the piston and hence the engine valve connected to the piston.
According to another preferred embodiment of the present invention, the control valve has a third possible position whereby the medium flow is intended to be directed so that it does not reach the power device. Switching the control valve from its first or second position to said third position prevents the medium reaching the first and second chambers. Movement of the piston and of the engine valve is thus halted. Preferably, the control valve in said third position also bars any flow to and from said first and second chambers. The piston and the engine valve are thus forcibly kept in their existing positions. Such switching to the third position may take place automatically when the engine valve has assumed a fully open or closed state. Alternatively, such switching may also take place when so-called partial lifting is desired, i.e. when it is not desired that the valve opens fully.
According to another preferred embodiment of the present invention, an outlet line is arranged in the cylinder and the medium flowing into the respective chamber is allowed to pass out via said outlet line when the piston has moved to a specific position in said chamber. Using such an outlet line means that the control valve requires only two positions for controlling the engine valve, thereby making it possible for the control valve to be provided with a short travel between the two positions. This may be an advantage, since the control valve has to be able to switch quickly in order, within a short space of time, to free large flow cross-sections. Such an outlet automatically establishes a circulating medium flow when the piston reaches an end position. With advantage, said outlet incorporates a throttle valve. Such a throttle valve provides the medium with a specific pressure in the outlet line. The pressure of the medium in the respective chamber in the cylinder thus becomes equal to that pressure. This medium pressure acts upon the piston so as to keep the engine valve in an open or closed position. Such a throttle valve may be settable so that the pressure in the outlet line can be varied.
According to a preferred embodiment of the present invention, the cylinder incorporates means intended to damp the piston""s movement in the cylinder when, or immediately before, it reaches an end position corresponding to a fully closed or open engine valve. As the piston requires a high velocity for switching the engine valve quickly, the piston requires damping at its end positions to prevent its being subject to excessive stresses. Said damping means may include at least part of the piston being provided with a cross-sectional area which decreases towards the end of the piston and is designed to be accommodated in a recess. The steadily decreasing cross-sectional area of the piston being led into a recess results in the medium enclosed in the recess by the end of the piston passing out through an aperture with a steadily decreasing cross-sectional area. In this way the piston can be provided with damping as it reaches its end position.
According to another preferred embodiment of the present invention, the control valve is designed to be controlled by electrical signals from a control unit. The control valve may incorporate solenoids which switch the control valve to a desired position according to electrical signals received from the control unit. The control unit emitting electrical signals may be a computer unit which uses information on the respective combustion engine to control the movements of the engine valves so as to achieve as close as possible to optimum operation of the engine on the basis of various operating parameters for the engine. Examples of how the engine can be controlled include facilities for exhaust braking, for alternate operation as a two-stroke or four-stroke engine, for using so-called internal EGR (whereby exhaust gases are intended to be retained in the cylinder before the next induction stroke) and for optimised operation with regard to economy or power requirements.
According to another preferred embodiment of the present invention, a retaining device is designed to keep the engine valve forcibly in a desired position. In cases where the circulating medium does not provide sufficient force to keep the engine valve in a closed position, such a retaining device can supply supplementary force as necessary. Such a retaining device may incorporate an electromagnet.